For a given airspeed, an aircraft may consume less fuel at a higher altitude than it does at a lower altitude. In other words, an aircraft may be more efficient in flight at higher altitudes as compared to lower altitudes. Moreover, bad weather and turbulence can sometimes be avoided by flying above such weather or turbulence. Thus, because of these and other potential advantages, many aircraft are designed to fly at relatively high altitudes.
As the altitude of an aircraft increases, from its take-off altitude to its “top of climb” or “cruise” altitude, the ambient atmospheric pressure outside of the aircraft decreases. Thus, unless otherwise controlled, air could leak out of the aircraft cabin causing it to decompress to an undesirably low pressure at high altitudes. If the pressure in the aircraft cabin is too low, the aircraft passengers may suffer hypoxia, which is a deficiency of oxygen concentration in human tissue. The response to hypoxia may vary from person to person, but its effects generally include drowsiness, mental fatigue, headache, nausea, euphoria, and diminished mental capacity.
Aircraft cabin pressure is often referred to in terms of “cabin altitude,” which refers to the normal atmospheric pressure existing at a certain altitude. Studies have shown that the symptoms of hypoxia may become noticeable when the cabin altitude is above the equivalent of the atmospheric pressure one would experience outside at 8,000 feet. Thus, many aircraft are equipped with a cabin pressure control system to, among other things, maintain the cabin pressure altitude to within a relatively comfortable range (e.g., at or below approximately 8,000 feet) and allow gradual changes in the cabin altitude to minimize passenger discomfort.
In addition to a cabin pressure control system, many aircraft also include an environmental control system (ECS) that supplies temperature-controlled ECS air to the aircraft cabin, which also improves passenger comfort. Typically, a flow of bleed air from one or more of the aircraft engines is supplied to the ECS, which in turn conditions the bleed air and supplies the ECS air to the aircraft cabin. The ECS air, when flowing into the aircraft cabin, will also pressurize the aircraft cabin and cause a change in cabin altitude. Thus, the cabin pressure control systems in such aircraft typically include at least an outflow valve and a controller. The outflow valve is mounted on the aircraft bulkhead and, when open, fluidly communicates the aircraft cabin to the environment outside of the aircraft. The controller implements various control laws and supplies appropriate valve control signals to the outflow valve that modulates the position of the outflow valve. As a result, the ECS air supplied to the aircraft cabin is controllably released from the aircraft cabin to the environment outside of the aircraft to thereby control aircraft cabin altitude.
In many instances, the aircraft cabin is not pressurized (relative to the surrounding environment) when the aircraft is on the ground. This allows the aircraft doors to be readily opened and closed to facilitate personnel ingress to and egress from the aircraft. However, once the aircraft is airborne, the cabin is controllably pressurized to attain a cabin altitude, via the cabin pressure control system, working in concert with the ECS, and remains controllably pressurized until the aircraft lands.
As may be appreciated, one of the functional goals of many cabin pressure control systems is to comfortably control cabin altitude during the aircraft take-off rotation. To meet this goal, the cabin pressure control system should comfortably control cabin altitude and cabin altitude rate of change as the aircraft altitude climbs from the initial take-off altitude, while the flow of air from the ECS system may be varying. One method that has been used to help meet this functional goal is to implement logic that pre-pressurizes the aircraft cabin (i.e., lowers cabin altitude) prior to take-off rotation. Such cabin pre-pressurization may be implemented by supplying ECS air to the cabin and commanding the outflow to the closed position.
Aircraft cabin pressure control systems, such as the ones described above, are robustly designed and manufactured, and are operationally safe. Nonetheless, these systems do suffer certain drawbacks. For example, in order to pre-pressurize the aircraft cabin, engine bleed air is needed so that the ECS can supply ECS air to the cabin. However, during the take-off roll in some aircraft, bleed air flow, and thus ECS air flow, may not be available, since the engines may need this additional air to increase engine power output. Moreover, if ECS air flow is not available during the take-off roll, it is subsequently reintroduced in order to properly control cabin altitude during the flight. Thus, the crew and passengers may experience two potentially uncomfortable and/or disconcerting changes in cabin altitude (e.g., cabin pressure), commonly referred to as “pressure bumps.” If the cabin pressure control system is implementing the above-mentioned cabin pre-pressurization logic without ECS air flow, the system will command the outflow valve to its closed (or near closed) position. However, since no ECS air is flowing into the cabin, cabin altitude may rise at the same rate as the aircraft, resulting in the first “pressure bump.” Thereafter, upon ECS air flow reintroduction, the aircraft cabin may begin pressurizing relatively quickly, which may cause an uncomfortably high cabin climb rate and the second “pressure bump.” The cabin pressure control system may additionally overcompensate for the high cabin climb rate, resulting in an uncomfortably high cabin descent rate. This fluctuation in cabin altitude rate may continue until the cabin pressure control system gains control.
Hence, there is a need for a cabin pressure control system that implements a control scheme that overcomes one or more of the above-noted drawbacks. Namely, a cabin pressure control system and method that at least reduces the magnitudes of the pressure bumps that may occur during aircraft take-off without bleed air flow and/or accommodates cabin altitude control during take-off both with and without bleed air flow. The present invention addresses one or more of these needs.